sm.c

static e_STATE current_state;

void main_thread()
{
    switch(current_state)
    {
        case STATE_A:
            state_a();
            break;
        ...
    }
}

void state_a()
{
    MESSAGE message;

    while(current_state == STATE_A)
    {
        receive_message(&message)
        switch(message.type)
        {
            case MESSAGE_A:
                //do a bunch of message a stuff...
                break;
            ...
        }
    }
}

...

sm.h

typedef enum {
    STATE_A,
    STATE_B,
    STATE_C
} e_STATE;